The present invention relates generally to the collection of radiant energy from solar sources. A particular type of solar collector is the cylindrical collector, which is formed by extending a transverse cross section longitudinally to form a trough like structure, examples of which are described in a publication, Solar Energy, Vol. 16, No. 2, pages 89-95 (1974), in a U.S. Letters Pat. No. 3,923,381 of Winston and in a prior application Ser. No. 581,613 filed May, 29, 1975. Such concentration devices take radiation which strikes an entrance aperture within a given angle of acceptance and concentrates it onto an exit aperture or onto the surface of an energy absorber positioned at the exit aperture with the area of the exit aperture or the surface of the absorber smaller than that of the entrance aperture. In the prior art cylindrical collectors the radiation reaching the exit aperture or the surface of the absorber is totally diffuse, i.e., it is spread out over all angles of incidence at the exit aperture or on the surface of the absorber less than 90.degree..
In the field of solar energy collection, there are many applications where large angles of radiation, i.e., near 90.degree., incident at the exit aperture or on the surface of the absorber are undesirable. For example, many energy absorber materials which might be positioned at the exit aperture have poor absorptivity at large angles of incidence. For a selective surface type absorber, such as a sandwich of aluminum oxide, molybdenum and aluminum oxide, absorptivity drops off dramatically at angles of incidence above 60.degree.. In applications where a glass plate is placed over the absorber to reduce convection, radiation incident at large angles is mostly reflected from the glass and does not reach the absorber. Some photovoltaic conversion type absorbers become reflective at large angles of incidence. If a portion of the reflected rays, which would otherwise be incident at the exit aperture or on the absorber at angles greater than desired, are directed within a desired range, increased efficiency in absorption may be obtained.
It is therefore an object of this invention to provide a device for collecting and concentrating solar radiation.
Another object of this invention is to provide a non-imaging, solar energy collection device with restricted angles of incidence of radiation at the exit aperture or on the surface as an energy absorber positioned at the exit aperture.